1. Field of the Invention
The present invention relates to a belt-type continuously variable transmission with a variable-speed pulley mechanism for use as a continuously variable force transmission in a vehicle, such as an automobile and, more particularly, to a transmission with a variable-speed pulley mechanism having a fluid actuated movable pulley part that is shifted axially relative to a fixed pulley part to change the width of a belt-receiving groove defined therebetween in rapid response to a variation in the rotational velocity of a shaft carrying the variable-speed pulley mechanism.
2. Background Art
Belt-type continuously variable transmissions for vehicles, such as automobiles, are known in the art. Typically, these transmissions employ a pair of variable-speed pulley mechanisms associated with spaced, parallel, input/drive and output/driven shafts. A V-belt, or other type belt, is trained around the variable-speed pulley mechanisms.
With the prior art variable-speed pulley mechanisms mounted on input and output shafts, force transmission is performed by varying the pitch diameters of the pulleys and, as a consequence thereof, the relative rotational velocities of the input and output shafts. In such systems, the output shaft may be used to drive accessories on automobiles, such as hydraulic pumps for power steering, alternators, etc.
Each variable-speed pulley has cooperating pulley parts defining variable width belt-receiving grooves. One of the cooperating pulley parts is normally fixed and the other axially movable relative thereto to set the width of the belt-receiving groove and the effective diameter of the variable-speed pulley.
The position of the movable pulley part is controlled in the art by a number of different mechanisms. It is known to provide discrete particles in a space bounded in part by the movable pulley part so that as the rotational velocity of the shaft increases, the centrifugal force on the discrete particles urges the particles radially outwardly and against the movable pulley part so that a component of that force shifts the movable pulley part relative to the fixed pulley part.
Typically, when the centrifugal-type shifting mechanism is employed on the variable-speed pulley on the input shaft, the cooperating output shaft has a variable-speed pulley with a movable pulley part that is spring biased towards a fixed pulley part.
An exemplary, centrifugal-type, variable-speed pulley mechanism is disclosed in Japanese Patent Publication No. 51-6815. In that structure, movable weights, such as steel balls, are placed in radial guide grooves and movable radially along the grooves in response to rotation of the variable-speed pulley.
The centrifugal-type transmission has several drawbacks. First, there is an insignificant axial force developed by the discrete particles at low speeds. On the other hand, with high speed rotation, the centrifugal force of the discrete particles increases in proportion to the square of the rotational speed of the pulley to produce oft times excessive forces, particularly when accessories are driven. The centrifugal-type transmissions are thus effectively operable only in relatively high speed environments. Accordingly, conventional variable speed pulley mechanisms operated by discrete particles moving under centrifugal force encounter difficulty when used at low speeds and often have undesirably large diameter pulleys. The pulley diameter on the input side is required to be quite large to allow gradual increase of the shifting force on the movable pulley part in response to an increase in the rotational velocity of the input shaft. Further, in the centrifugal-type variable pulley mechanisms, due to the resistance encountered in axially shifting the movable pulley parts against the belt to vary the width of the groove, there is a lag in the movement of the movable pulley such that the curve on the graph plotting variation of the output speed with respect to that of the input speed at the time of acceleration significantly differs from the corresponding curve during deceleration i.e. there is a resulting hysteresis loss. In a vehicle in which there is a hysteresis loss, different output speed is obtained for the same input speed depending on whether the engine is accelerating or decelerating, which presents a problem in safety as well as operating performance.
In a centrifugal-type variable speed pulley mechanism, when the rotation of the input shaft increases, the rotational speed of the output shaft also increases. Accordingly, the rotational speed of the accessories, such as the hydraulic pump for controlling power steering, and the like, operated by the output shaft, also increases. While accessories are not generally adversely affected at lower speeds, excessive rotational forces at higher rotational speeds may cause rapid deterioration of the accessories and the belts connecting the accessories to the output shaft. There is also a significant energy loss resulting from the input shaft rotating at excessively high speeds.
Alternatively, hydraulic pressure can be utilized to shift the movable pulley parts towards the fixed pulley parts. An exemplary structure is shown in U.S. Pat. No. 4,601,680. In that structure, the amount of oil supplied to effect shifting of the movable pulley parts is dictated by an electronic control unit.
There has recently been developed a continuously variable transmission mechanism in which a fluid reservoir rotates together with a rotary shaft. The variation of the flow speed of the fluid in the reservoir, indicative of the shaft speed, is detected by a pitot tube to control the position of the movable pulley parts. One exemplary structure is shown in Japanese Patent Laid-Open No. 59222660.
Another continuously variable transmission is disclosed in UK Patent No. 1,525,674, in which a pitot tube is provided for sensing the increase in the rotational velocity of the shaft. The shaft carries a fluid reservoir in which the pitot tube is immersed. A pump and hydraulic control valve communicate with the pitot tube to generate a fluid pressure to move a movable pulley part relative to a fixed pulley part to thereby adjust the width of the belt-receiving groove.
In those transmissions utilizing hydraulic pressure, the amount of oil delivered is normally controlled by an electronic controller. A large amount of data is input to an electronic control unit. Several sensors are required for the data to be generated and the unit to operate effectively. The resulting structures are quite complicated and resultingly expensive, particularly when the hydraulic system interconnects both the input and output shafts.